We have isolated a "resting" intermediate in P2 phage DNA replication from a P2-phage-infected E. coli mutant. This intermediate is bound to the "outer cell membrane" and contains one continuous circular DNA strand, as well as one interrupted strand. The interruption is completely strand-specific. Our objectives are: 1) to show that the interruption occurs at the replication origin, 2) to define the chemical structure of the discontinuity, and 3) to determine whether this intermediate carries bound to it the cisacting product of P2 gene A. The head, tail and lysis genes (late genes) of temperate phage P2 can be activated by two different mechanisms, one of which operates during normal infection, and another, which occurs when satellite phage P4 infects a P2-lysogenic cell. We are attempting to elucidate these control mechanisms by isolating P2 and P4 early gene products which bind to RNA polymerase. In the course of these studies we discovered a P4- induced transcribing activity which may initiate P4 DNA replication. The head of satellite phage P4 is composed mostly of P2 head protein, but the P4 head is only 1/3 the size of the P2 head. We have discovered a P4-specific late protein which is found in the P4 head (about 45 copies), and which may determine head size. We can assay in vitro for head size determination, using a DNA packaging assay.